1. Field of the Invention
The present invention relates to a driving device for liquid crystal display panel of a passive drive system and to a liquid crystal display device and an electronic device employing the same.
2. Background Information
A passive-drive liquid crystal display panel widely used in electronic devices has a structure wherein a liquid crystal layer is sealed between a transparent plate formed with a plurality of line-shaped common electrodes and a transparent counter plate formed with a plurality of line-shaped segment electrodes. Individual intersections of the common electrodes and the segment electrodes define individual pixels which constitute a display screen. That is, the display screen of m×n pixels is essentially composed of m segment electrodes and n common electrodes. ON/OFF control of each pixel is accomplished by way of a signal (driving signal waveform) applied to the segment electrode and the common electrode. Such a liquid crystal display device has a problem that an observer cannot instantaneously determine whether a pixel is OFF because it is located outside a display screen or because an OFF signal is inputted thereto. In other words, a boundary between the display screen and a non-display area is unrecognizable for the observer unless an outermost portion of the display screen (displayable area) is illuminated. For example, a commonly used color liquid crystal panel is black in an OFF state (with no voltage applied). Therefore, when a photo or game graphic with a black background is displayed on such a panel, the outermost portion of the display screen (display area) is unrecognizable. This may lead to a demand for a so-called frame display wherein outermost edges of the display screen are constantly illuminated. On the other hand, the current cellular phones or portable information terminals are generally limited in the number of display pixels in order to expedite the display of information provided via network services or by add-on applications. As to the portable information terminal, for instance, the number of display pixels depends upon an OS used therein. Normally, the OS does not give considerations to the frame display. Hence, the following method is commonly used to provide the frame display outside the display screen of m×n pixels based on the conventional passive-drive system. That is, additional pixels for frame display are provided outside the display screen so that (m+2)×(n+2) pixels in total are arranged. A graph-plotting capability is used to draw a frame while a primary display image is written to a window (display screen of m×n pixels) inside the frame. Specifically, SSD1780, a conventional liquid crystal driver commercially available from SOLOMON Systech Limited is capable of writing frame data on a respective line on vertically and laterally opposite sides externally of the display screen by utilizing a 2D graph-plotting capability of a display controller, which is called 2D graphic Limitation Graphic Acceleration (refer to, for example, non-patent document 1). However, commonly used liquid crystal drivers and controllers are not equipped with such a capability so that liquid crystal panels adapted for the frame display based on this method are limited. In addition, there is another problem that the IC having such a capability is more expensive than the common ICs. [Non-patent Document 1] The specification of the liquid crystal driver SSD1780 commercially available from SOLOMON Systech Limited (SSD1780-0.15 Full Version Specification, P42, Item93.1–93.6) retrieved on Sep. 26, 2002 via internet service (URL http://www.solomon-systech.com/products/product_lists.htm).
As described above, the attempt to provide the frame display outside the display screen can be accomplished only when the specific controller or driver is used. This imposes restriction on the specification of the liquid crystal display device. In addition, a significant cost must be borne in order to utilize the IC having such a capability.
In view of the foregoing, it is an object of the invention to permit any controller or driver to provide the frame display outside the display screen in an easy and less-costly manner without increasing the scale of the driving circuit.